1. Field of the Invention
The present invention relates to a method for removing copper oxide on the surface of a copper film for wiring and a method for patterning a copper film, and more particularly to a method for removing copper oxide naturally or artificially formed on the surface of a copper wiring used in a printed wiring board or an LSI and a method for patterning a copper film.
2. Description of the Prior Art
Copper is widely used as a wiring material since it is low electrical resistance and easy to process and inexpensive in cost.
However, copper has a demerit that it is easily oxidized in the air to form copper oxide on the surface of a copper film. When copper is used as a wiring material, therefore, the wiring is liable to become high in resistance or liable to break because of copper oxide formed on the surface of the wiring.
On the other hand, it is described in Japanese Patent Laid-open disclosure (KOKAI) Sho 58-11789, Sho 63-52499, Sho 3-217077, Hei 4-151898, and the like to positively form copper oxide on the surface of a copper film in order to improve adhesion of photoresist or resin to a copper film.
However, since copper and copper oxide are different from each other in such chemical properties as solubility, etching characteristic, and the like, existence of a copper oxide layer causes a wiring pattern to be deteriorated in accuracy in case of making a wiring through patterning a copper film by means of a photolithography method.
Copper oxide is soluble in both of alkali and acid. Therefore, when patterning of a copper film, the copper oxide existing under a resist film to act as a mask for the copper film is dissolved in an etchant to deteriorate the wiring pattern in accuracy. And in case of plating of a copper wiring, it causes the wiring pattern to be deteriorated in accuracy due to the copper oxide formed on the surface of the copper wiring, which dissolves in the plating solution.
It is said that oxidation of the surface of a copper film progresses even in a high vacuum and a copper oxide film of 20 to 30 .ANG. in thickness is formed in a usual vacuum condition.
When the inventors of the invention actually measured thickness of copper oxide films formed on the surface of copper films which have been just formed by vacuum deposition by means of an X-ray photoelectron spectroscope, any sample was not found whose copper oxide film was less than 20 .ANG. in thickness.
However, as a usual condition of photolithography for making a wiring of a copper film, generation of a copper oxide film of 20 to 30 .ANG. in thickness is no problem.
However, actually the growth of copper oxide (film) makes more progress or advances in general so that it becomes greater than 30 .ANG. in thickness without stopping at a degree of thickness of a copper oxide film made under a condition of high vacuum.
Such oxidation of copper as this is also daily seen. For example, such oxidation is made easy to understand by recalling a fact that a copper coin which has an intense metallic gloss immediately after being issued becomes reddish brown as the days pass. Actually copper coins are made of copper alloy, but their oxidation behavior is similar to copper.
Oxidation of copper progresses faster in case that the copper is placed in a high-temperature and high-humidity environment, in case that there is acid or alkali on the surface of the copper or in the atmosphere around it, or in case that there is hydrogen sulfide, sulfur dioxide, high-density carbon disulfide, or the like in the atmosphere around the copper.
A method of etching the surface of copper with acid is generally used as a method for removing copper oxide film on the surface of the copper. Many examples using such a method in printed copper wiring boards are actually seen.
However, since acid is intensely adsorbed onto the surface of a copper film due to high activity of copper, it is difficult to completely remove the residual acid on the surface of the copper film by means of simple washing with water. The residual acid not only acts as a catalyzer which prompts corrosion or oxidation of the copper itself but also causes other materials around it to be corroded. Therefore, even if the copper oxide has been removed with acid, oxidation of the surface of the copper film is progressed or advanced still further after removal of the copper oxide. This is similar to a fact that a reddish brown copper coin takes back a metallic gloss when it is washed with acid but after this it is more rusted than before washing.
In case of forming a copper wiring, therefore, even when patterning a copper film after removing copper oxide on the surface of the copper film with acid and then washing its surface with water, such a failure that the copper wiring becomes high electrical resistance or is broken happens after the patterning.